What is the difference between tanks , vessels & drums. One thing comes to my mind is the storage capacity tanks have much larger storage capacity , second is vessel & third is drum. Are there other differences ?

I would not waste any time lingering on semantics. The important thing to always bear in mind is that there are Pressure Vessels (a generic term) and atmospheric vessels.

The pressure vessels are of major importance because, as an engineer, you are responsible for protecting the public against a rupture due to over-pressure or process upset. However, the atmospheric vessels - especially storage tanks - can be hazardous to humans as well.

Reactors are a special breed of pressure vessels because they can generate their own pressure and are also susceptible to run-away reactions. They therefore require specific and careful control mechanisms and operations.

Boilers and direct-fired heaters are still another special variety of pressure vessels. There are special codes that deal with them because of the special problems they present.